The process of marking a registration mark on a photoreceptor (PR) during the first pass of a xerographic color printer, and then using that mark on subsequent passes to enable proper registration of the other color toner layers.
It is common in a xerographic printer to use a flexible belt photoreceptor or a belt on which toner is deposited before transfer to paper to increase flexibility of configuration of charging modules, raster output scanner (ROS), developer station, etc. Also, the net length available is much greater compared to a circular drum. Therefore several stations in series along the belt where each station deliveres one color becomes possible. In a black and white system, this works well, but in a color printing system there is the added difficulty, because of this flexibility, of guaranteeing that all of the layers of toner, usually four, are perfectly registered with respect to one another in spite of the slight expansion, contraction, twisting and velocity gradients of the belt. Therefore, the exact position and area of the previous layers of toner must be known before the next layer of toner can be deposited. Various methods of determining this have been proposed.
U.S. Pat. No. Re. 32,967 and U.S. Pat. No. 3,701,464 use a series of rectangular perforations adjacent one or both edges of the web as tracking indicia. In the alternative, these marks may be preprinted on the web. These marks can then be read out using a light source and photosensors. In operation, the marks are sensed and the placement of the toner on the web is calculated. By definition, any discrepancy between the location of the registration marks and the actual location of the toner will go undetected since the toner itself is not used as the indicia. Also, their correction method is mechanical through the stepper motors action on the rollers.
In U.S. Pat. No. 2,802,666 four registration marks, one in each of four colors of ink, are placed on the paper web between pages printed in color. The marks are shaped and positioned so that if one pass is depositing ink that is slightly out of registration with the others, this displacement will be detected and corrected for subsequent pages.
All of the above systems rely on an iterative process whereby successive pages are monitored and web distortions are corrected for over a period of time, and over a number of prints. A problem arises in a typical color copier where once the first primary color toner layer is deposited, the second and subsequent passes for the next color primaries must result in their being placed exactly on top of the first pass color primary toner layer, wherever this first pass toner happens to be placed.
An additional problem in a laser printer is that the scan speed, as determined by the polygon speed of rotation, is also slightly variable. Typically, start of scan (SOS) and end of scan (EOS) detectors will be used. The scanning beam will illuminate these at the start and end of the scan, and the beam velocity can be calculated therefrom. However, these devices will not accurately determine where the toner is actually being deposited, and that must be known if the later passes are to deposit toner in registration with the first.